Specific antisera to C1r. Quantitation of C1r in normal and pathological human sera

Monospecific neutralizing and precipitating antisera to C1r, a subunit of the first complement component, were obtained. These antisera neutralized C1r activity in purified preparations and in macromolecular C1 and did not react with C1q or C1s. They formed one line of precipitation in the β-globulin region with normal human serum, C1hu and C1r at various stages of purification. Using anti-C1r antiserum and a radial immunodiffusion technique, the concentration of C1r was determined in normal, SLE and RA sera. It was 101 μg/ml in normal sera and lower in sera of active SLE patients (69·7 μg/ml). No significant variations from normal were found in sera of SLE patients in periods of remission or in RA patients.